1. Field of the Invention
The present invention relates to a process for the production of a silver-palladium alloy fine powder that may be used for the production of electrode materials, contacts or the like used in the electronic industries field, in particular for the production of ceramic capacitors or the like.
2. Description of the Prior Art
In the electronic industries field, silver-palladium pastes have been widely used in thick film conductive circuits employed in screen printing. The objects of using silver-palladium alloys are to repress the migration occurring when wiring is made by the use of silver alone, to raise the melting point of silver by adding palladium so that the conditions for using substrates at desired temperatures are applicable, and the like.
In recent years, with the small size and precision of electronic parts, there have been growing demands for decreasing the thickness of conductive films to several microns and narrowing the width of and between wiring circuits (to, for instance, 100 microns). To meet such demands, it is required that the conductive metal powder consistuting the paste should have a small particle diameter, an even grain size and a uniform composition.
In contrast with this, the fact is that the silver-palladium paste has usually been used in the manner of mixing separately manufactured silver powder and palladium powder for a very long period of time and then judging their mutual uniform dispersion.
This process has the following problems:
(1) A very long period of time such, for instance, as several tens of hours is needed for the dispersion of the powders, and in addition thereto,
(2) The heterogeneity in composition in the microscopic region cannot be avoided.
Accordingly, the demands for said electronic parts can never be completely satisfied.
In addition, the use of the usual silver-palladium mixture for the inner electrode of the ceramic capacitors has been followed by such disadvantages that delamination is very likely to be caused between the electrode and the ceramic and the capacity of the capacitor is likely to be lowered. This is because said mixed powder undergoes abnormal expansion and shrinkage during the alloy formation during the heat treatment which is conducted at temperatures lower than the burning point of the ceramic to increase its strength, namely in the range of 100.degree.-500.degree. C. This tendency becomes more violent as the film thickness of dielectrics becomes thinner.
As a way to solve the above problems, it has been proposed to use a silver-palladium alloy powder. Some reports thereon have been made. For instance, there are enumerated Japanese Laid Open Patent Application No. 11701/1983, Japanese Laid Open Patent Application No. 132503/1984 and Japanese Laid Open Patent Application No. 224111/1984. From the viewpoint of the powder manufacturing process there is enumerated Japanese Laid Open Patent Application No. 56125/1978.
However, the abovementioned prior art processes for the production of silver-palladium alloy powders are comprised of heat-reducing a mixed oxide or mixed hydroxide of silver and palladium in a stream of hydrogen (Japanese Laid Open Patent Application No. 11701/1983, Japanese Laid Open Patent Application No. 132503/1984 and Japanese Laid Open Patent Application No. 56125/1978) or of mixing the silver powder and palladium powder and then heating same at 300.degree. C. in air (Japanese Laid Open Patent Application No. 224111/1984).
However, these processes are not industrially profitable because of their needing of precursors to be alloyed, namely oxide, hydroxide or 2 kinds of metal powders, using dangerous hydrogen, and the like. Further, heating to high temperatures in air is not preferable because the particles are mutually sintered, and the silver and palladium are oxidized. In addition these prior art processes have such problems as the powder prepared by said process being partially alloyed and coexistent with silver and a palladium powder, whereby delamination can never be prevented completely.